Process for the preparation of expandable polystyrene by continuous injection of a liquid organic peroxide

ABSTRACT

A process for the preparation of expandable polystyrene including the following steps: i°) heating an aqueous suspension including styrene monomer and at least one organic peroxide initiator of formula (I) 1-alkoxy-1-t-alkylperoxycyclohexane in which the alkoxy group contains 1 to 4 carbon atoms, the t-alkyl group contains 4 to 12 carbon atoms, and the cyclohexane ring may optionally be substituted with 1 to 3 alkyl groups each, independently having 1 to 3 carbon atoms, at a temperature ranging from 100° C. to 120° C.), ii°) adding a blowing agent selected from the group of alkanes having from 4 to 6 carbon atoms and mixtures thereof. Also, an expandable polystyrene obtainable according to such a process and to insulation parts and packaging including such an expandable polystyrene.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. applicationSer. No. 13/386,958, filed on Feb. 17, 2012, which is a national stageapplication of International application No. PCT/FR2010/051548, filed onJul. 22, 2010, which claims priority of French application no. 0955241,filed on Jul. 27, 2009, and U.S. provisional application No. 61/228,740,filed on Jul. 27, 2009. Each of U.S. application Ser. No. 13/386,958,International application no. PCT/FR2010/051548, French application no.0955241, and U.S. provisional application No. 61/228,740 are herebyincorporated by reference in entirety.

TECHNICAL FIELD

The present invention relates to a process for the preparation ofexpandable polystyrene and to an expandable polystyrene obtainable bysuch a process.

BACKGROUND

A known method for producing expandable polystyrene polymers,hereinafter referred to as EPS, is by aqueous suspension polymerization.It is typically a batch process where two or more monomer-solublepolymerization initiators are used with a rising stepwise, continuous,or combination temperature profile. Initiators for the process areselected on the basis of their half-life temperatures to provide ameasured supply of radicals at selected points along the temperatureprofile such that effective conversion occurs within an acceptableperiod of time. For styrene polymerization, it is convenient to describeinitiator decomposition performance in terms of one hour half-lifetemperature, defined as that temperature sufficient to causedecomposition of one half of the starting concentration of initiatorover a one hour time period.

Traditionally, suspension polymerization to prepare EPS is conducted ina process using two different temperature stages and two initiators, afirst stage initiator and a second stage or “finishing” initiator, withdifferent half-life temperatures, each appropriate for the particulartemperature stage. In such a process, dibenzoyl peroxide (BPO) is oftenused as the first stage initiator at a reaction temperature of about 82°C. to 95° C. Other first stage initiators useful in this temperaturerange might include tertiary butyl peroxy-2-ethylhexanoate, tertiaryamyl peroxy-2-ethylhexanoate and2,5-dimethyl-2,5-di(2-ethylhexanoylperoxy)hexane. Initiators such astertiary butyl peroxybenzoate (TBP) or dicumyl peroxide (DCP) are widelyused for the higher temperature stage, or second stage, at 115° C. to135° C. The second stage is usually a finishing step intended tominimize residual monomer in the EPS. In commercial processing, thisstage is often held above 125° C. for prolonged intervals to reducemonomer content to acceptable levels.

Characteristic shortcomings of the traditional process are long reactiontimes necessary to obtain adequate conversion in the first stage andrelatively high finishing temperatures required in the second stage.

In view of alleviating this drawback, document U.S. Pat. No. 6,608,150has proposed an improved process for the preparation of expandablepolystyrene in which “intermediate” temperature peroxides, that is tosay initiator peroxides having a one hour half-life temperature rangingfrom 101° C. to 111° C., are used. According to this document, the useof such “intermediate” peroxides in the preparation of expandablepolystyrene allows to reduce the conversion time for some hours.

Expandable polystyrene, as prepared in the suspension process, is in theform of essentially spherical beads. It is mentioned in U.S. Pat. No.6,608,150 that some blowing agent, such as pentane, may be used in theprocess in order to render the polystyrene beads expandable.

However, it is known that the use of pentane in a process of preparationof expandable polystyrene leads to a decrease in the molecular weight asa result of transfer mechanism on the alkane agent (see Principles ofPolymerization-George Odian p 203-217, McGraw-Hill Book Company 1970).

Additionally, it is more and more requested in the industry that thepolystyrene resins comprise flame retardant additives for safetyreasons. For instance, flame retarded resins are widely used ininsulation applications. The addition of such additives may also lead toa decrease of the molecular weight of the resulting polystyrene.

It appears then that several factors may lead, in the preparationprocess of expandable polystyrene, to a significant decrease of themolecular weight of such expandable polystyrene.

BRIEF SUMMARY OF THE DISCLOSURE

By “molecular weight” is meant according to the present application theweight average molecular weight (Mw). According to the presentinvention, the molecular weight is measured according to the SEC (SizeExclusion Chromatography) method using Polystyrene standards.

It is important that the expandable polystyrene has a high molecularweight in order to guarantee the mechanical properties of the finalmaterials.

The Applicant has now discovered that by using a specific initiatorperoxide, it was possible to accelerate the preparation of expandablepolystyrene while using a blowing agent and to still obtain polystyrenehaving a particularly high molecular weight.

The Applicant has also discovered that the continuous addition of saidspecific initiator peroxide during the polymerization reaction, can alsoaccelerate the preparation of expandable polystyrene.

DETAILED DESCRIPTION OF THE DISCLOSURE

A first aspect of the invention relates to a process for the preparationof expandable polystyrene comprising the following steps:

I°)a) preparing an aqueous suspension comprising styrene monomer

I°)b) heating the suspension at a polymerization temperature rangingfrom 100° C. to 120° C.,

I°)c) adding continuously, before, during and/or after step I°)b) atleast one organic peroxide initiator of formula (I)1-alkoxy-1-t-alkylperoxycyclohexane in which the alkoxy group contains 1to 4 carbon atoms, the t-alkyl group contains 4 to 12 carbon atoms, andthe cyclohexane ring may optionally be substituted with 1 to 3 alkylgroups each, independently having 1 to 3 carbon atoms,

ii°) adding a blowing agent selected from the group consisting ofalkanes having from 4 to 6 carbon atoms and mixtures thereof.

The process according to the invention allows obtaining expandablepolystyrene showing high molecular weight. Preferably, the expandablepolystyrene obtainable according to the process of the invention shows amolecular weight (Mw) of at least 170,000 g/mol, preferably of at least175,000 g/mol, and more preferably of at least 190,000 g/mol.

Because of its high molecular weight, the expandable polystyreneaccording to the invention may be used efficiently in insulation orpackaging applications.

Moreover, the process of the invention has a conversion time reducedcompared to the ones of the prior art.

The process of the invention uses at least one organic peroxideinitiator of formula (I), i.e. 1-alkoxy-1-t-alkylperoxycyclohexane inwhich the alkoxy group contains 1 to 4 carbon atoms, the t-alkyl groupcontains 4 to 12 carbon atoms, and the cyclohexane ring may optionallybe substituted with 1 to 3 alkyl groups each, independently having 1 to3 carbon atoms.

According to the present invention, it has been found that the use ofsuch a specific organic peroxide of formula (I) in combination with ablowing agent such as an alkane having from 4 to 6 carbon atoms andmixtures thereof allowed to obtain expandable polystyrene having a highmolecular weight, in a short time. Such a result could not be obtainedwith other organic peroxides, having a one hour half-life temperaturesimilar to that of compounds of formula (I) but having a chemicalformula different than formula (I).

As organic peroxides of formula (I) one can cite1-alkoxy-1-t-amylperoxycyclohexane and1-alkoxy-1-t-hexylperoxycyclohexane, where the alkoxy group contains 1to 4 carbon atoms, and the cyclohexane ring may optionally besubstituted with 1 to 3 alkyl groups each, independently having 1 to 3carbon atoms.

In an embodiment of the invention, the at least one organic peroxideinitiator is 1-methoxy-1-t-amylperoxycyclohexane (also referredhereinafter as TAPMC).

In a preferred embodiment of the process according to the invention thepolymerization mixture is formulated at a temperature below the reaction(polymerization) temperature and subsequently heated to reach saiddesired reaction temperature. In such a process preferably at most 40%by weight (% w/w), more preferably at most 30 to 20% by weight, and mostpreferably at most 5% by weight of the organic peroxide, based on thetotal weight of the peroxide used during the polymerization, is presentbefore step I°)b) at the start of the heating-up phase, while theremainder is added continuously over a period of at least 1, preferably2, and more preferably 2-4 hours during or after step I°)b), dependingon the polymerization time. More preferably, the remainder of theperoxide is added from the time the reaction mixture temperature iscontrolled at the desired reaction temperature.

The use of a small amount of peroxide from the start allows a fastheating up and start of the polymerization, since this peroxide willalready (partly) decompose during the heating of the polymerizationmixture. When the polymerization mixture reaches the polymerizationtemperature, the remainder of the peroxide can be added to the mixtureto control the further polymerization rate. Preferably, the addition iscontinuous, since this allows the most accurate control of thepolymerization rate and a constant polymerization heat output, ensuringthe highest efficiency and polystyrene quality. The addition time of 2-4hours allows a very efficient use of the initiator. By using suchaddition times, high yields of high-quality polystyrene were attained.

In another preferred embodiment, the reaction mixture is formulated ator near the polymerization temperature. In this process, hereinaftercalled warm-start process, it is not necessary to add a certain amountof peroxide at the start while the remainder is dosed over time.However, also in this warm-start process it can be beneficial to add upto 40% w/w of all peroxide immediately after formation of the reactionmixture, the remainder being added over time. Also in this warm-startprocess preferably at least 10% w/w of all peroxide is present from themoment the reaction mixture reaches the desired reaction(polymerization) temperature. If this procedure is used, the peroxideaccording to the invention preferably is added as the last ingredient.This procedure is particularly preferred if a certain amount ofpolymerization inhibitor (a radical trapping species) is present in thereaction mixture. If such a radical scavenger is present, for instancebecause it is introduced with the monomer wherein it is typically usedas a stabilizer, the initially dosed peroxide will react with saidscavenger, thus preventing a delayed start of the polymerizationreaction.

The organic peroxide initiator of formula (I) is used in a first stepi°) of the process of the invention, during which said organic peroxideinitiator is heated at a polymerization temperature ranging from 100° C.to 120° C. In an embodiment of the invention, this temperature rangesfrom 105° C. to 115° C., preferably is 110° C. Such a temperature isclose to the one hour half-life temperature of the organic peroxides offormula (I), as measured according to the well-known technique ofmeasuring the rate of initiator decomposition in the aromatic solventcumene. Indeed, in a preferred embodiment of the invention, the organicperoxides of formula (I) are used as first stage initiators.

During the step I°)a), the aqueous suspension is heated, at apolymerization temperature ranging from 100° C. to 120° C., preferably105° C. to 115° C., and more preferably of 110° C. for a time sufficientto effect at least partial decomposition of said initiator and initiatepolymerization of the styrene monomer. This time may range from about1.5 hours to about 3 hours.

It is to be understood that the words “continuous addition” is used todescribe the step of adding peroxide to the polymerizing reactionmixture at polymerization conditions.

The addition can be done intermittently during the polymerization over aperiod of time wherein preferably at least 20%, preferably at least 40%,more preferably at least 60%, of all monomer used in the process ispolymerized, meaning that at least two portions of initiator are addedto the reaction mixture, or it can be continuous, meaning that for acertain period of time the initiator is continuously added to thereaction mixture, or any combination of these techniques. Examples of acombination of such techniques include, for instance, a process whereinthe initiator is first added continuously, then the addition is stopped,and then again it is added continuously. If an intermittent operation isselected, there are at least 2, preferably at least 4, more preferablyat least 10, and most preferably at least 20 moments at thepolymerization temperature at which the initiator is added.

Most preferably, the peroxide is added continuously and/orintermittently from the start of the polymerization reaction, preferablyafter at least 5%, more preferably at least 10%, even more preferably atleast 20%, most preferably at least 30%, of the monomer(s) has alreadybeen polymerized. During the addition period at least 2, preferably atleast 5, more preferably at least 10%, more preferably at least 20%,more preferably at least 30%, and most preferably at least 50%, of allmonomer used in the process is polymerized. The addition can be effectedat any suitable entry point to the reactor.

In an embodiment of the invention, the aqueous suspension of step i°)further comprises at least one additional organic peroxide initiator,different from said organic peroxide initiator of formula (I).

Preferably, said additional organic peroxide initiator plays the role ofthe second stage initiator. When the aqueous suspension of step i°)comprises an organic peroxide initiator of formula (I) as first stageinitiator and at least one additional other organic peroxide initiatoras second stage initiator, step i°) comprises a stage I°)b′), duringwhich said suspension is heated at a temperature ranging from 100° C. to120° C., preferably from 105° C. to 115° C., and more preferably to 110°C., and a second stage I°)c′) during which said suspension is heated ata temperature corresponding to the one hour half-life temperature of theat least additional other organic peroxide.

Said additional peroxide initiator may be selected among any organicperoxide initiator different from formula (I) and having a one hourhalf-life temperature, as measured according to the well-known techniqueof measuring the rate of initiator decomposition in the aromatic solventcumene, above 110° C. In an embodiment of the invention, the additionalperoxide initiator is selected from the group consisting of compoundshaving the formula (II), i.e. OO-t-alkyl-O-alkyl monoperoxycarbonate,wherein t-alkyl contains from 4 to 12 carbon atoms, preferably from 4 to5 carbon atoms, and alkyl contains from 3 to 12 carbon atoms, andpreferably 8 carbon atoms, and their mixtures.

In an embodiment of the process of the invention, the at leastadditional peroxide initiator is OO-t-amyl-O-2-ethylhexylmonoperoxycarbonate (also referred to hereinafter as TAEC).

In another embodiment of the process of the invention, the at leastadditional peroxide initiator is OO-t-butyl-O-2-ethylhexylmonoperoxycarbonate (also referred to hereinafter as TBEC).

These products are respectively commercially available from the companyARKEMA under the trade name “Luperox® TAEC” and “Luperox® TBEC”.

In an embodiment of the process of the invention, in particular whenOO-t-amyl-O-2-ethylhexyl monoperoxycarbonate orOO-t-butyl-O-2-ethylhexyl monoperoxycarbonate is used as the secondstage initiator, the aqueous suspension is heated, during said secondstage I°)b′), at a temperature ranging from 120° C. to 140° C.,preferably at a temperature of about 130° C. Said second stage may lastfrom 1 to 3 hours.

In an embodiment of the process of the invention, the second stageinitiator is added continuously, as the first stage initiator.

In an embodiment of the process of the invention, said first stage lastsabout 2 to 3 hours and said second stage lasts about 2 hours.

In another embodiment of the process of the invention, the organicperoxide of formula (I) plays the role of the second stage initiator andsaid additional peroxide initiator plays the role of the first stageinitiator.

In an embodiment of the invention, said organic peroxide initiator offormula (I) is used in the aqueous suspension of step i°) in amountsfrom 4 to 25 milli equivalents of initiator per liter of styrene, morepreferably from 12 to 20 milli equivalents of initiator per liter ofstyrene.

Styrene is the preferred monomer for the process of the invention.However, up to 15% of the weight of styrene may be replaced by otherethylenically unsaturated copolymerizable monomers such asalkylstyrenes, alpha methylstyrene, acrylic acid esters andacrilonitrile. For example, the styrene monomer to be polymerized mayalso contain up to 15 weight %, with respect to the total weight ofstyrene, of copolymerizable monomers other than styrene monomers.

In an embodiment of the invention, said additional organic peroxideinitiator used in the aqueous suspension of step i°) in amounts from 1to 20 milli equivalents of initiator per liter of styrene, morepreferably from 2 to 10 milli equivalents of initiator per liter ofstyrene.

The process of the invention also comprises a step, step ii°), ofaddition of a blowing agent selected from the group consisting ofalkanes having 4 to 6 carbon atoms and mixtures thereof.

The blowing agent may be added to the aqueous suspension at any timeduring step i°), or after step i°) is completed.

In an embodiment of the invention, the blowing agent is added duringstep i°), i.e. during the polymerization of the styrene monomer. In sucha case, when step i°) comprise a first and a second stages, the blowingagent may be added during the first stage of step i°). For instance, theblowing agent may be added at the end of the first stage of step i°).

In another embodiment of the invention, the spherical beads ofpolystyrene obtained at the end of step i°) are segregated by size andthe blowing agent is added to the segregated spherical beads. Thisembodiment of the process allows a more precise control of the beadsizes.

In an embodiment of the invention, said blowing agent is selected fromthe group consisting of butane, 2-methylbutane, pentane, cyclohexane andmixtures thereof. Such blowing agents are well known products which arecommercially available.

Preferably, said blowing agent is pentane. In an embodiment of theinvention, the blowing agent is added in amounts ranging from 5 to 10%,by weight, with respect to the weight of styrene.

In an embodiment of the invention, an additive selected from the groupcomprising flame retardant agents, inorganic suspension stabilizers,such as calcium phosphate or magnesium pyrophosphate, organic suspensionstabilizers, such as polyvinylpyrrolidone, polyvinyl alcohol orhydroxyethylcellulose, surfactants, chain transfer agents, nucleatingagents, expansion aids, lubricants, plasticizers and mixtures thereof,is added to the aqueous suspension at step i°) or at step ii°).

The flame retardant agent may be selected from the group comprisinghexabromocyclododecane (HBCD), .tetrabromobisphenol A (TBBPA),decabromodiphenyl ether (Deca-BDE), pentabromodiphenyl ether(Penta-BDE), octabromodiphenyl ether (Octa-BDE),tris-(dibromopropyl)phosphate, carbon tetrabromide,beta-dibromopropionate, tetrabromoethylene,1-2-dibromo-1,1,2,2-tetrachloroethane, 1,1,2,2-tetrabromoethane,dibromodichloroethane, 1,2-dibromo-1,1-dichloroethane,1,2-dibromo-1,2,2-trichloroethane, 1,2,3,4-tetrabromobutane,1,2,3-tribromopropane, pentabromoethane, tribromotrichlorocyclohexane,1,2,4-tribromobutane, tetrabromopentane, hexabromoethane,tetrabromodichlorocyclohexane, pentabromomonochlorocyclohexane,1,2-di-(dibromomethyl)benzene, alpha,beta-dibromoethylbenzene,alpha,beta-dibromopropionate and mixtures thereof.

Preferably, the flame retardant agent is hexabromocyclododecane. Theperoxide or peroxides used in the process according to the invention areadded to the aqueous suspension at step i°) in the pure form or,preferably, in the form of a dilute solution or dispersion (such as asuspension or emulsion in styrene). One or more suitable solvents can beused to dilute the initiator. Solvents are of such a nature that it isacceptable to leave them as a residue in the final polymer, as it is thecase for solvents that are desired plasticizers for the final resin.

Another aspect of the invention relates to an expandable polystyreneobtainable by the process described above. Such an expandablepolystyrene shows a high molecular weight and is therefore useful ininsulation or packaging applications. For example, the expandablepolystyrene according to the invention may be used in the manufacture ofinsulation parts, for example in the transport industry. The expandablepolystyrene according to the invention may also be used in themanufacture of packaging having good mechanical properties. A furtheraspect of the invention relates to insulation parts comprising anexpandable polystyrene according to the invention. Another aspect of theinvention relates to packaging comprising an expandable polystyreneaccording to the invention.

The present invention will now be further illustrated by means of thefollowing examples.

EXAMPLES

In all examples below, the molecular weight of the product obtained ismeasured according to the method given hereinabove in the description ofthe present application.

Example 1

Preparation of 1-methoxy-1-t-amylperoxycyclohexane (TAPMC)

A mixture of t-amyl hydroperoxide (TAHP), cyclohexanone and methanol istreated with 70% sulfuric acid at −6° to −4° C. In fifteen minutes, anequilibrium mixture of 1-methoxy-1-t-amylperoxycyclohexane,1,1-di-(t-amylperoxy)-cyclohexane and the unreacted starting materialscyclohexanone and TAHP is formed. Small amounts (≈2%) ofcyclohexane-1,1-dimethyoxyketal (CDMK) are also produced in the reactionmixture. The reaction mixture is quenched with cold water and theaqueous phase is separated from the organic phase, which is purified bywashing.

Example 2 (Comparative) BPO/TAEC

Into a 2 liter pressure vessel of the Büchi type, equipped with a doubleenvelop and specific stirring means (3 blade stirrer on two levels) wereadded at 20° C. and under stirring 680 g of deionized water with 0.64 gof polyvinyl alcohol (Alcotex® 72.5 available from the company HARCO).Were further added 320 g of styrene monomer with 1.44 g of dibenzoyleperoxide (Luperox® A75 available from the company ARKEMA), and 0.42 g of00-t-amyl 0-2-ethylhexyl monoperoxycarbonate (TAEC available under thetrademark Luperox® TAEC from the company ARKEMA). This aqueoussuspension was heated at 90° C. for 1 hour and maintained at thistemperature for 4 hours for the first polymerization stage. In thisexample, dibenzoyle peroxide is used as the first stage initiator.

After 5 hours, were further added 21.1 g of pentane in two minutes usinga high pressure pump. The reaction medium was then heated at 130° C. for0.5 hour and maintained at this temperature for 2 hours. In thisexample, OO-t-amyl O-2-ethylhexyl monoperoxycarbonate is used as secondstage initiator.

The reaction medium was then cooled down 1 hour. The obtainedpolystyrene beads are collected by filtration and dried.

The obtained product has a molecular weight (Mw) of 158,000 g/mol with970 ppm of residual styrene monomer.

The overall cycle time is 8.5 hours.

Example 3 (Comparative) TAPMC/TAEC

Into a 2 liter pressure vessel of the Büchi type, equipped with a doubleenvelop and specific stirring means (3 blade stirrer) were added at 20°C. and under stirring 680 g of deionized water with 0.64 g of polyvinylalcohol (Alcotex® 72.5 available from the company HARCO). Were furtheradded 320 g of styrene monomer with 1.02 g of1-methoxy-1-t-amylperoxycyclohexane (TAPMC) as obtained in Example 1above, and 0.42 g of OO-t-amyl O-2-ethylhexyl monoperoxycarbonate (TAECavailable under the trademark Luperox® TAEC from the company ARKEMA).This aqueous suspension was heated at 110° C. for 1 hour and maintainedat this temperature for 2.5 hours for the first polymerization stage. Inthis example, 1-methoxy-1-t-amylperoxycyclohexane is used as first stageinitiator.

After 3.5 hours, were further added 21.1 g of pentane in two minutesusing a high pressure pump. The reaction medium was then heated at 130°C. for 0.5 hour and maintained at this temperature for 2 hours. In thisexample, OO-t-amyl O-2-ethylhexyl monoperoxycarbonate is used as secondstage initiator.

The reaction medium was then cooled down during 1 hour. The obtainedpolystyrene beads are collected by filtration and dried.

The obtained product has a molecular weight (Mw) of 226,000 g/mol with700 ppm of residual styrene monomer.

The overall cycle time is 7 hours.

Example 4 (Comparative) TAPMC/TBEC

The same process as in Example 3 is repeated with the same amounts andthe same cycle time except that the 0.42 g of TAEC is replaced by 0.42 gof OO-t-butyl O-2-ethylhexyl monoperoxycarbonate (Luperox® TBECavailable from the company ARKEMA).

In this example, 1-methoxy-1-t-amylperoxycyclohexane is used as firststage initiator and OO-t-butyl O-2-ethylhexyl monoperoxycarbonate isused as second stage initiator.

The obtained product has a molecular weight (Mw) of 220,000 g/mol with500 ppm of residual styrene monomer.

Example 5 (Comparative) HBCD

The same process as in Example 3 is repeated with the same amounts andthe same cycle time except that 2.1 g of hexabromocyclododecane is addedat the aqueous suspension of styrene monomer.

The obtained product has a molecular weight (Mw) of 153,000 g/mol with1600 ppm of residual styrene monomer.

Example 6 (Invention) TAPMC/TAEC Continuous Injection

Into a 2 liter pressure vessel of the Büchi type, equipped with a doubleenvelop and specific stirring means (3 blade stirrer on two levels) wereadded at 20° C. and under stirring 680 g of deionized water with 0.64 gof polyvinyl alcohol (Alcotex® 72.5 available from the company HARCO).Were further added 320 g of styrene monomer with 0.42 g of OO-t-amylO-2-ethylhexyl monoperoxycarbonate (TAEC available under the trademarkLuperox® TAEC from the company ARKEMA). This aqueous suspension washeated at 110° C. while simultaneously adding continuously at a constantflow rate using a pressure pump a solution made up of 1.02 g of1-methoxy-1-t-amylperoxycyclohexane (TAPMC) and 26.2 g of styrene.Duration of the injection is 2.5 hours for the first step of thepolymerization.

After 2.5 hours, were further added 21.1 g of pentane in two minutesusing a high pressure pump. The reaction medium was then heated at 130°C. for 0.5 hour and maintained at this temperature for 2 hours. In thisexample, OO-t-amyl O-2-ethylhexyl monoperoxycarbonate is used as secondstage initiator.

The reaction medium was then cooled down for 1 hour. The obtainedpolystyrene beads are collected by filtration and dried.

The obtained product has a molecular weight (Mw) of 206,000 g/mol with764 ppm of residual styrene monomer.

The overall cycle time is 6 hours.

Example 7 (Invention) TAPMC/TAEC Continuous Injection

Into a 2 liter pressure vessel of the Büchi type, equipped with a doubleenvelop and specific stirring means (3 blade stirrer on two levels) wereadded at 20° C. and under stirring 680 g of deionized water with 0.64 gof polyvinyl alcohol (Alcotex® 72.5 available from the company HARCO).Were further added 320 g of styrene monomer with 0.42 g of OO-t-amylO-2-ethylhexyl monoperoxycarbonate (TAEC available under the trademarkLuperox® TAEC from the company ARKEMA). This aqueous suspension washeated at 115° C. while simultaneously adding continuously at a constantflow rate using a pressure pump a solution made up of 1.02 g of1-methoxy-1-t-amylperoxycyclohexane (TAPMC) and 20.8 g of styrene.Duration of the injection is 2 hours for the first step of thepolymerization.

After 2 hours, were further added 21.1 g of pentane in two minutes usinga high pressure pump. The reaction medium was then heated at 130° C. for0.5 hour and maintained at this temperature for 1 hour. In this example,OO-t-amyl O-2-ethylhexyl monoperoxycarbonate is used as second stageinitiator.

The reaction medium was then cooled down for 1 hour. The obtainedpolystyrene beads are collected by filtration and dried.

The obtained product has a molecular weight (Mw) of 195,000 g/mol with940 ppm of residual styrene monomer.

The overall cycle time is 4.5 hours.

Example 8

(Invention) TAPMC/TAEC/HBCD continuous Injection

The same process as in Example 6 is repeated with the same amounts andthe same cycle time except that 2.1 g of hexabromocyclododecane is addedat the aqueous suspension of styrene monomer.

The obtained product has a molecular weight (Mw) of 193,000 g/mol with770 ppm of residual styrene monomer.

The overall cycle time is 6 hours.

1. Process for the preparation of expandable polystyrene comprising thefollowing steps: I°)a) preparing an aqueous suspension comprisingstyrene monomer; I°)b) heating the suspension at a polymerizationtemperature ranging from 100° C. to 120° C.; I°)c) adding continuously,during and/or after step I°)b) at least one organic peroxide initiatorof formula (I) 1-alkoxy-1-t-alkylperoxycyclohexane in which the alkoxygroup contains 1 to 4 carbon atoms, the t-alkyl group contains 4 to 12carbon atoms, and the cyclohexane ring may optionally be substitutedwith 1 to 3 alkyl groups, each independently having 1 to 3 carbon atoms,at most 40% by weight (% w/w) of the organic peroxide based on the totalweight of the peroxide used during the polymerization, is present beforestep I°)b), while the remainder is added continuously over a period ofat least 1 hour during or after step I°)b); and ii°) adding a blowingagent selected from the group consisting of alkanes having from 4 to 6carbon atoms and mixtures thereof.
 2. Process according to claim 1,wherein the at least one organic peroxide initiator is1-methoxy-1-tamylperoxycyclohexane (TAPMC).
 3. Process according toclaim 1, wherein said blowing agent is selected from the groupconsisting of butane, 2-methylbutane, pentane, cyclohexane and mixturesthereof.
 4. Process according to claim 1, wherein the temperature ofstep i°) ranges from 105° C. to 115° C.
 5. Process according to claim 1,wherein the aqueous suspension of step i°) further comprises at leastone additional organic peroxide initiator, different from said organicperoxide initiator of formula (I).
 6. Process according to claim 5,wherein said additional peroxide initiator is of formula (II)OO-t-alkyl-O-alkyl monoperoxycarbonate, wherein t-alkyl contains from 4to 12 carbon atoms and alkyl contains from 3 to 12 carbon atoms andtheir mixtures.
 7. Process according to claim 1, wherein said organicperoxide initiator of formula (I) is used in the aqueous suspension ofstep i°) in amounts from 4 to 25 milli equivalents of initiator perliter of styrene.
 8. Process according to claim 1, wherein the styrenemonomer to be polymerized also contains up to 15 weight %, with respectto the total weight of styrene, of copolymerizable monomers other thanstyrene monomers.
 9. Process according to claim 1, whereinhexabromocyclododecane is added to the aqueous suspension at step i°) orat step ii°).
 10. Process according to claim 5, wherein the aqueoussuspension of step i°) comprises the organic peroxide initiator offormula (I) as a first stage initiator and the at least one additionalother organic peroxide initiator different from said organic peroxideinitiator of formula (I) as a second stage initiator, step i°) comprisesa stage I°)b′), during which said suspension is heated at thetemperature ranging from 100° C. to 120° C., and a second stage I°)c′)during which said suspension is heated at a temperature corresponding tothe one hour half-life temperature of the at least additional otherorganic peroxide different from said organic peroxide initiator offormula (I).
 11. Process according to claim 1, wherein 20 to 30% byweight of the organic peroxide is added continuously during step—I°)c)—.
 12. Process according to claim 1, wherein, at most, 5% byweight of the organic peroxide is added continuously during step—I°)c)—.
 13. Process according to claim 1, wherein the organic peroxideis added continuously over a period of at least 2 hours.
 14. Processaccording to claim 1, wherein the organic peroxide is added continuouslyover a period of 2-4 hours.
 15. Process according to claim 4, whereinthe wherein the temperature of step i°) is 110° C.
 16. Process accordingto claim 6, wherein the t-alkyl contains from 4-5 carbon atoms and thealkyl contains 8 carbon atoms.
 17. Process according to claim 7, whereinthe organic peroxide initiator of formula (I) is used in the aqueoussuspension of step i°) in amounts from 12 to 20 milli equivalents ofinitiator per liter of styrene.
 18. Process according to claim 10,wherein the suspension is heated at a temperature ranging from 105° C.to 115° C.
 19. Process according to claim 10, wherein the suspension isheated to a temperature of 110° C.